Owners of rooftop solar installations are bracing for substantially higher monthly electricity bills under a proposal by San Diego’s utility monopoly to overhaul the way it charges customers of all kinds.

San Diego Gas & Electric asked regulators this month for permission to bill customers separately for use of its distribution grid.

The effect on nonsolar customers would be negligible, but unbundling grid use from the cost of electricity would fundamentally change the energy-saving equation for homeowners and businesses with rooftop solar installations.

How much solar customers would pay remains unclear — bills will depend greatly on each ratepayer’s use on the grid.

The proposed “network use charge” is pegged to the ebb and flow of electricity for those who generate excess power by day only to draw from the grid when the sun doesn’t shine.

The utility has framed its request to the California Public Utilities Commission as an issue of equity and fairness. As more solar is adopted, SDG&E says, traditional customers are footing more of the bill for infrastructure and public purpose programs, which subsidize low-income customers and pay for energy efficiency and utility research and development.

“We have to have a rate structure that is sustainable and can accommodate those (solar) customers,” said J.C. Thomas, manager of government and regulatory affairs at SDG&E.

Feeling betrayed

Many of the 14,500 ratepayers who generate their own power feel they’ve been betrayed, as they calculate a longer payoff time on their solar investments.

Patricia and Gil Field, a substitute teacher and a retired Navy contracting officer, installed solar panels in 2009 on their Carmel Valley home, using a combination of inheritance money, a loan from their Quaker meeting group and discounts based on a state rebate and federal tax incentives.

“They’re changing the rules and charging an excessive rate to undercut people who have made environmentally sound commitments,” Patricia Field said of the SDG&E proposal.

Analysts say SDG&E’s application has the attention of California’s two other investor-owned utilities, Southern California Edison and Pacific Gas and Electric, which could seek to replicate it.

Solar customers remain a tiny fraction of SDG&E’s 1.4 million customers. But that market segment is growing at a frantic pace and represents mostly heavy energy users — once the most profitable of utility customers.

“For the really forward-looking utilities, there is the writing on the wall that they’re really not in the business of being the primary energy source for these people,” said Ted Ko, associate executive director of the nonprofit Clean Coalition in San Francisco. “This proposal by SDG&E is kind of a first attempt at … the compensation model for people who are going to be generating their own energy.”

Net-metering incentive

Perhaps more than any other incentive, one simple equation has helped turn rooftop solar panels into a mainstream reality in San Diego County.

Under “net metering,” utility customers who generate one kilowatt hour of energy can buy one less kilowatt hour from the utility.

The solar payoff also has been sweetened by California’s steeply tiered rate structure, a legacy of the energy crisis a decade ago that saw rolling blackouts amid electricity market manipulation.

To comply with state law, California’s utilities charge heavy users of electricity nearly twice the rate per kilowatt hour to encourage conservation and assure cheap energy to cover the most basic needs.

As more solar is adopted, at a current rate of 200 customers a month, the number of customers paying upper-tier rates has declined, according to SDG&E.

Top-tier customers, the company says, in effect are already paying $34 a year to offset costs for solar customers.

“We don’t think it’s fair that just because you can put solar on your roof that your neighbor should pay for it,” SDG&E spokeswoman Stephanie Donovan said.

Navy vet: ‘It’s a bad trend’

Despite assurances from SDG&E that grid-use charges won’t eliminate the benefits of net metering, clean-energy contractors fear for their future under the proposal, according to Andrew McAllister, director of programs at the ratepayer-funded California Center for Sustainable Energy.

“Decreasing the effectiveness of the incentives embedded in net metering is going to have a chilling effect on the marketplace” for small-scale solar, McAllister said.

Retired Navy pilot Rusty Chang, 55, of Chula Vista had anticipated that lower monthly utility bills would pay for his rooftop solar installation over eight years. Now he knows it could take longer to produce real savings.

“I have no idea how long. That’s the problem,” Chang said. “It would definitely push back the time. If that’s the trend SDG&E wants to follow, it’s a bad trend.”

The rate redesign is already being challenged before regulators.

The San Diego-based Utility Consumers’ Action Network says it amounts to an obscured surcharge against self-generating electricity customers, and would circumvent state law that provides for rate stability and predictability.

Initial hearings on the application are scheduled for December. SDG&E hopes to win approval by mid-2012 and roll out the new billing structure in 2013. New charges would be phased in by 2015.

The utility bill restructuring is part of SDG&E’s larger general rate case proposal to raise gas and electric rates by $1.1 billion over the next four years.

SDG&E says the new network-use charge would be offset by lower electricity rates. A $3 monthly basic service fee also would be implemented.

People with electric vehicles would be eligible for relief from grid-use charges to encourage charging during predawn hours when the demand for electricity is lowest.